How to create old looking/ cracked glass
Here, try this:

As Eric pointed out in the comments, the secret here is a Voronai texture set to &quot;distance to edge&quot;. I used ColorRamps to remove all the grayscale and leave only black outlines. I made a large, medium, and small set of cracks and mixed them together. I then used this combo as a main image (BaseColor), as a basis for a normal map, and I also inverted it to make an alpha mask.

Final result looks like this:

This can obviously be made better by creating more distortion among the cracks, but I feel this is a good place to start. I'll upload the .blend file so you can experiment.
File is here - 
Hope this helps.